The present invention relates to alkali silicate glasses, and more particularly to alkali silicate and alkali borosilicate glasses exhibiting low thermal coefficients of elastic modulus in combination with good glassworking characteristics.
As is well known, glasses share a common characteristic with most other solid materials in exhibiting an elastic modulus which varies depending upon the temperature of the glass. As is also known, the extent of this dependence can vary substantially depending upon the composition of the glass and the temperature regime over which the elastic modulus of the glass is measured. Thus V. S. Postnikov et al., in "Temperature Dependence of the Shear Modulus and Attenuation of Ultrasound in Silicate Glasses," Izv. Akad. Naukd. SSSR, Neorg. Matr., 7 [3] pages 485-489 (1971) report a large positive thermal coefficient of shear modulus with temperature for fused quartz, and also describe glasses in the SiO.sub.2 --K.sub.2 O--ZnO--BaO--PbO composition system which exhibit relatively low thermal coefficients of shear modulus over the temperature range -160.degree. to +160.degree. C.
Glasses exhibiting a low thermal coefficient of elastic modulus are useful for a variety of applications wherein glasses which do not change in elasticity with changes in ambient temperature are needed. Examples of products requiring glasses with a low thermal coefficient of elastic modulus include those wherein the acoustic propagation characteristics of the glass are critical, and those wherein the detection and measurement of a deflection in a glass element is used to provide an indication of a mass or applied stress.
Specific uses for glasses with temperature-invariant properties include their use to make glass components for inertial balances such as described, for example, by H. Patashnick and G. Rupprecht in "Microweighing Goes On Line in Real Time," Research and Development, pages 74-78 (June 1986). In this application, the elastic modulus of the glass is critical because an attached mass is determined from the frequency of a vibrating glass mass-collecting element. Changes in the elastic modulus of this element with temperature also change the vibration frequency and thus introduce error into the weighing process.
While some of the glasses disclosed by Postnikov et al. exhibit relatively temperature-independent elastic modulus behavior, these glasses are generally quite unstable and therefore very difficult to melt and form into products without devitrification. Thus to our knowledge no commercial application has been found for these glasses.
Commercial glasses which inherently exhibit relatively low thermal coefficients of elasticity are also known. U.S. Pat. No. 2,392,314, for example, describes glasses consisting essentially of 60-75% SiO.sub.2, 10-20% B.sub.2 O.sub.3, 10-15% BaO and/or CaF.sub.2, and 5-10% Al.sub.2 O.sub.3, and also comprising an alkali metal oxide component including up to about 5% K.sub.2 O, up to about 2% Li.sub.2 O, and up to 3% Na.sub.2 O.
Glasses of this type were originally developed for sealing to cobalt nickel alloys. A related glass, Corning Code 7052 glass, which is commercially sold for this application, exhibits a relatively low thermal coefficient of elastic modulus at ambient temperatures. Further, in contrast to the glasses of Postnikov et al., this glass exhibits excellent working characteristics.
Unfortunately, however, even these commercial compositions do not exhibit very low thermal coefficients of elastic modulus. Hence glasses having thermal coefficients on the order of 20% or less of that of Corning Code 7052 glass would be preferred for many of these applications.
Desirable characteristics of glasses to be used for technical applications such as above described further include, in addition to acceptable glass-working behavior, good dimensional stability, a low coefficient of thermal expansion, and good chemical durability.
It is therefore a principal object of the present invention to provide glasses of novel composition which exhibit lowered thermal coefficients of elastic modulus in combination with good glass working characteristics, high dimensional stability, a low thermal expansion coefficient and good chemical durability.
Other objects and advantages of the invention will become apparent from the following description thereof.